This invention relates generally to processes for producing ingots of reactive metals and alloys and more particularly to a process for producing homogeneous ingots several times larger than those which can be produced from a single induction skull melted charge.
Melting of reactive metal alloys containing Titanium, Zirconium, or other reactive elements is usually done in a vacuum, in an inert atmosphere, or in a partial pressure of gas which is non-reactive with the alloy constituents. Such non-reactive atmosphere melting preferably employs induction melting or arc melting in a cold crucible or mold, usually of water-cooled copper, to eliminate contamination from mold washes or from the mold itself. These melting methods, both of which are very well known in the metallurgical art, produce ingots of excellent purity; however, induction melting and vacuum arc melting each have limitations which result in ingots having less than ideal properties.
Induction skull melting (ISM) produces ingots having a very high degree of cleanliness and homogeneity due to melting in a skull of the alloy being processed and to the intense stirring produced in the melt by the induction field. However, the size of induction skull melted heats is limited by the presence of the water-cooled metal crucible and the size of the controlled atmosphere chamber. The crucible limits the strength of the induction field, and the chamber limits the size of the mold and the ability to manipulate the crucible for pouring the alloy into the mold. This results in small ISM ingotsxe2x80x94typically less than 100 pounds. Consequently, for large ingots, several ISM heats must be combined. This may be accomplished by casting several small electrodes from several ISM heats, welding the electrodes together end-to-end, and using the resulting composite electrode for vacuum arc remelting (VAR) the alloy in a water-cooled copper crucible.
However, since hot-topping during casting is not possible in the ISM melting and casting process, the resulting ingots have at least some porosity and shrinkage pipe. These defects aggravate the costliness and difficulty of producing the VAR electrode by welding. Further, since many of the reactive alloys are brittle, they also have marginal weldability and are susceptible to cracking in the welds and in the heat affected zones adjacent to the welds. Therefore, the welded composite electrodes may contain cracks and inclusions due to contamination with oxygen and nitrogen during welding. This can lead to failure of the electrodes during VAR and can result in damage to the equipment and danger to the operators thereof.
Vacuum arc melting can produce very large ingots compared to those made by ISM. Electrodes of, for example, titanium alloys for vacuum arc melting are typically made by starting with titanium sponge and/or granular master alloys and/or alloying elements, which are blended together in required proportions and compacted into briquettes of about 4xe2x80x3 diameter and 2xe2x80x3 thickness. The briquettes are non-homogeneous because the alloying elements are introduced as solids and are only mechanically blended. The electrode is formed by welding the briquettes together, usually using titanium welding wire, under controlled atmosphere conditions. This is a costly and time consuming process, and, at best, produces an electrode which lacks homogeneity and may include weld defects.
Typically, the electrode is melted in a vacuum arc furnace using a water-cooled copper crucible having a diameter slightly larger than that of the electrode. The resulting ingot is non-homogeneous due to the non-homogeneous electrode and to the lack of stirring in the arc melting process, and it usually contains unmelted or partially melted granules of some starting materials. The ingot is used as an electrode for VAR in a water-cooled copper crucible again having a diameter slightly larger than that of the electrode to produce a second-stage ingot. This ingot is the electrode for a third stage VAR process which produces a final triple-melted ingot. Ingots of titanium alloys, made by the VAR process, may be as large as 16,000 pounds, and although they are clean, due to vacuum melting, and free of porosity and pipe, due to the hot-topping capability of arc melting, they are non-homogeneous and may still contain oxygen and nitrogen enriched inclusions due to the welding required to produce the starting electrode. Moreover, vacuum arc melting and VAR limits alloy compositions due to the difficulty of alloying some materials by mechanically mixing and the lack of stirring during arc melting.
The foregoing illustrates limitations known to exist in present methods for producing large ingots of reactive metal alloys. Thus it would be advantageous to provide an alternative directed to overcoming one or more of those limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by making a homogeneous electrode of reactive metal alloy, comprising an axially disposed unitary core made from one induction melted heat of said reactive metal alloy and having a diameter xe2x80x9cdxe2x80x9d and a length xe2x80x9cLxe2x80x9d; and a body having an outer diameter at least 2 times xe2x80x9cdxe2x80x9d and length xe2x80x9cLxe2x80x9d, said body being disposed about said core, and comprising at least one induction melted heat of said reactive metal alloy, said homogeneous electrode being vacuum arc remelted to produce a homogeneous ingot.
The foregoing and other aspects of the invention will become apparent from the following detailed description, when considered in conjunction with the accompanying drawings.